The effects of season and dietary salt content on body temperature daily rhythms of common spiny mice from different micro-habitats

We compared body temperature (T(b)) daily rhythms in two populations of common spiny mice, Acomys cahirinus, during summer and winter months in relation to increasing dietary salt content. Mice were collected from the North and South facing slopes (NFS and SFS) of the same valley, that are exhibiting mesic and xeric habitats, respectively. During the summer, whilst mice were offered a water source containing 0.9% NaCl, SFS individuals had T(b) peak values at 24:00, whereas NFS individuals had peak values at 18:00. When the salinity of the water source was increased, from 0.9 to 2.5% and then 3.5%, the difference between maximal and minimal T(b) of both populations increased. In addition, with increased salinity, the T(b) daily peak of SFS mice shifted to 18:00. During the winter, the mean daily T(b) values of both populations of mice were lower than during the summer. At 0.9% salinity, the NFS mice exhibited a daily T(b) variation with a peak at the beginning of the night. However, we did not detect any significant variation in daily T(b) in the SFS mice. At 2.5% salinity, the difference between the mean daily T(b) of mice from the two slopes increased. In winter we were unable to increase the salinity to 3.5% as the animals began to lose weight rapidly. We suggest that common spiny mice that inhabit these two micro-habitats are forming two discrete populations that respond differently to the environmental pressures prevailing in each habitat, by evolving different physiological capacities.     

A seasonal difference of daily energy expenditure in a free-living subterranean rodent,the silvery mole-rat (Heliophobius argenteocinereus; Bathyergidae)

In seasonal climatic regimes, animals have to deal with changing environmental conditions. It is reasonable to expect that seasonal changes are reflected in animal overall energetics. The relation between daily energy expenditure (DEE) and seasonally variable ecological determinants has been studied in many free-living small mammals; however with inconsistent results. Subterranean mammals, i.e. fossorial (burrowing) mammals which live and forage underground, live in a seasonally and diurnally thermally stable environment and represent a suitable model to test seasonality in DEE in respect to seasonal changes, particularly those in soil characteristics and access to food supply. Both factors are affected by seasonal rainfall and are supposed to fundamentally determine activity of belowground dwellers. These ecological constraints are pronounced in some tropical regions, where two distinct periods, dry and rainy seasons, regularly alternate. To explore how a tropical mammal responds to an abrupt environmental change, we determined DEE, resting metabolic rate (RMR) and sustained metabolic scope (SusMS) in a solitary subterranean rodent, the silvery mole-rat, at the end of dry season and the onset of rainy season. Whereas RMR did not differ between both periods, mole-rats had 1.4 times higher DEE and SusMS after the first heavy rains. These findings suggest that rainfall is an important environmental factor responsible for higher energy expenditure in mole-rats, probably due to increased burrowing activity. SusMS in the silvery mole-rat is comparable to values in other bathyergids and all bathyergid values rank among the lowest SusMS found in endothermic vertebrates.     

Exploring Phenotypic Plasticity in the LichenRamalina capitata: Morphology, Water Relations and Chlorophyll Content in North- and South-facing Populations

The present work analyses the morphology, anatomy, water relationsand chlorophyll content of thalli of the lichenRamalina capitatavar.protectafromtwo different populations exposed to contrasting microclimaticconditions due to differences in the orientation of the rocksurface. The population on the north-facing rock surface (NFS)was exposed to lower photosynthetic photon flux densities (PPFD),remained at high relative humidities for longer periods of timeand was exposed to lower temperatures than the population onthe south-facing surface (SFS). We proposed the hypothesis thatthe shadier the habitat the greater the ecological advantagefor enhanced light harvesting. Thalli from the SFS had shorterand wider lacinia, thicker thalli, mostly due to increased medullathickness, a higher water-retention capacity, a higher percentageof thallus volume occupied by the algal cells and a higher chlorophyllcontent than thalli from NFS. The phenotypic plastic responseof the traits studied inR. capitatavar.protectawas not directlyrelated to differences in the light availability, at least forthe range of PPFD experienced by the two populations studied,since the population exposed to higher PPFD exhibited largeramounts of light harvesting pigments. Both populations exhibitedthe same intrathalline distribution of algal cells and chlorophylls,which were more abundant in the apical than in the basal zonesof all thalli studied. Periods of water-induced metabolic activitywere shorter in the SFS than in the NFS, and structural andchlorophyll data indicated that thalli from the SFS were betterprepared for the photosynthetic exploitation of these brieferperiods and for maintaining thallus hydration into dry periods.These results suggest that differences in selective pressurebetween the two populations ofR. capitatavar.protectastudiedinvolved maximization of the photosynthetic exploitation ofthe periods of metabolic activity when they are brief, as hasbeen described for certain vascular plants from xeric environments. Ramalina capitatavar.protecta; algal cells; chlorophylls; water relations; microclimate; morphology; intrathalline variation; lichen; phenotypic plasticity     

Cost of living in free-ranging degus (Octodon degus): seasonal dynamics of energy expenditure

Animals process and allocate energy at different seasons at variable rates, depending on their breeding season and changes in environmental conditions and resulting physiological demands. Overall total energy expenditure, in turn, should either increase in some seasons due to special added demands (e.g. reproduction) or it could simply remain at about the same level, in which case the animals must show compensatory rebalancing of other expenditures that can be reduced. To test for the alternative hypotheses of seasonal variability or compensation, we measured total daily energy expenditure (DEE) in free-living degus (Octodon degus) at four seasons and followed this with determinations of basal metabolic rate (BMR) in the laboratory in the same individuals. DEE varied seasonally but was only significantly different (lower) in summer (non-breeding season), with a DEE:BMR ratio of only 1.6, whereas autumn, winter and spring DEE values were statistically indistinguishable from one another and showed DEE:BMR ratios ranging from 1.9 to 2.2. Our values of DEE in the field fall within the broad range of allometric expectation for herbivorous mammals in general, but the ratios of DEE:BMR are lower than expected. This, together with the lack of strong major shifts in total levels of DEE, suggests that degus are showing compensatory shifts among various categories of energy expenditure that allow them to manage their overall energy balance by minimizing total expenditure.     

Seasonal variation in energy expenditure and body composition in captive White Storks (Ciconia ciconia)

North Western European populations of White Storks (Ciconia ciconia) appear to have been saved from extinction by settling, i.e. stopping migration. Settled storks exposed to winter conditions must cope with periods of potentially high energy demands that would otherwise be avoided by the migration process. Doubly labeled water (DLW) was therefore used to examine the seasonal variation (summer vs winter) in daily energy expenditure (DEE) and the body composition of adult and immature storks of both sexes. Male White Storks showed a higher DEE over the winter period than in summer compared with females; in particular, immature males exhibited greater energy expenditure in winter than adult males. Thus, the DEE did not significantly differ between summer and winter (except for immature males), reflecting an absence of thermoregulation cost in winter. For both age classes, total body mass increased in winter, which was mainly due to an increase in fat mass. Adult storks were 5% heavier than immature storks. The sexes differed in body mass, with males weighing significantly more than females by 11%. Mean LBM (lean body mass) was 8.5% higher in adults than in immatures, and was 11.5% higher in males compared with females. Between their first and second summers, immatures accumulated a lean body mass to finally reach the same values as adults, indicating a phase of muscle development. The mean fat mass of the storks did not differ between age classes or between sexes. Based on physiological parameters, this study shows that settled White Storks are able to cope with mild winter periods when they are artificially provided with food. In a view to preserve favourable habitats for this species, it is therefore necessary to decide on a plan of action for breeding areas.     

Gastropod biodiversity at the 'Evolution Canyon' microsite, lower Nahal Oren, Mount Carmel, Israel

Twenty-six species of gastropods (terrestrial, shell-bearing snails and slugs) were recorded at the 'Evolution Canyon' microsite, lower Nahal Oren, Mount Carmel, Israel. Twenty-five species were recorded at the temperate, mesic north-facing slope (NFS) and 20 species at the xeric south-facing slope (SFS). Out of these species, six were NFS specific ( Pilorcula raymondi hebraica , Euchondrus septemdentatus , Monacha crispulata , Pyramidula rupestris hierosolymitana , Truncatellina haasi , and Vitrea contracta ) and one was SFS specific ( Prolimax eustrictus ). The interslope difference was probably partly due to missing forest species at the SFS in comparison with the NFS. Twenty-two species were Levantine endemics (84.6%) and four species were more widely distributed in the Palaearctic region (15.4%). The Levantine species are inhabitants of the mesic and mainly mountainous regions, but four species ( Granopupa granum , Calaxis hierosolymarum , Cecilioides acicula , and Helix engaddensis ) also penetrate the deserts. Seven species ( Buliminus labrosus , H. engaddensis , Levantina spiriplana caesareana , Metafruticicola fourousi , Monacha syriaca , Sphincterochila cariosa , and Xeropicta vestalis joppensis ) were significantly more abundant on the SFS than on the NFS. The local physical microclimatic sharp divergence leads to gastropod adaptive interslope biotic divergence caused by natural selection.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 147–155.     

Do slope orientation and sampling location determine soil biota composition?

"Evolution Canyon" is a typical Mediterra-nean-basin canyon with a summer dry stream at the bottom and large differences in vegetation cover and solar radiation between the north-facing slope (NFS) and the south-facing slope (SFS). It is known to act as an abiotic mediator influencing the community structure of soil fauna. The aim of this study was to determine the spatial dispersion of soil microbial and free-living nematode communities in the open sites (between shrubs) in the upper (0-10 cm) soil layer at four altitudes on both slopes. The combination of relative soil moisture availability and temperature, known to be one of the main triggers for soil biota activity, was explained by slope orientation. The above-mentioned differences were found to significantly affect microbial biomass and CO2 evolution resulting in temporary stress, corresponding to higher values in metabolic quotient (qCO2) values. These differences may represent microbial investment in energy in order to overcome stress resulting from the microclimatic differ-ences between the two slopes. Moreover, the degree of substrate limitation (primary production due to the differences in plant cover) of microbial activity was explained by the difference in microbial functional groups. The total abundance of soil free-living nematodes was found to be 2-fold higher on the SFS than on the NFS. Thirty-nine genera, including 12 bacteria-feeders, 5 fungi-feeders, 12 plant-parasites, and 10 omnivore-predators, were found at the study site, with 34 genera on the NFS and 29 on the SFS. The generic diversity of the bacteria-feeding nematodes was higher on the SFS than on the NFS. This study elucidates the importance of slope orientation and its effect on the structural levels of soil microbial and nematode communities.     

Do slope orientation and sampling location determine soil biota composition?

“Evolution Canyon” is a typical Mediterranean-basin canyon with a summer dry stream at the bottom and large differences in vegetation cover and solar radiation between the north-facing slope (NFS) and the south-facing slope (SFS). It is known to act as an abiotic mediator influencing the community structure of soil fauna. The aim of this study was to determine the spatial dispersion of soil microbial and free-living nematode communities in the open sites (between shrubs) in the upper (0–10 cm) soil layer at four altitudes on both slopes. The combination of relative soil moisture availability and temperature, known to be one of the main triggers for soil biota activity, was explained by slope orientation. The above-mentioned differences were found to significantly affect microbial biomass and CO₂ evolution resulting in temporary stress, corresponding to higher values in metabolic quotient (qCO₂) values. These differences may represent microbial investment in energy in order to overcome stress resulting from the microclimatic differences between the two slopes. Moreover, the degree of substrate limitation (primary production due to the differences in plant cover) of microbial activity was explained by the difference in microbial functional groups. The total abundance of soil free-living nematodes was found to be 2-fold higher on the SFS than on the NFS. Thirty-nine genera, including 12 bacteria-feeders, 5 fungi-feeders, 12 plant-parasites, and 10 omnivore-predators, were found at the study site, with 34 genera on the NFS and 29 on the SFS. The generic diversity of the bacteriafeeding nematodes was higher on the SFS than on the NFS. This study elucidates the importance of slope orientation and its effect on the structural levels of soil microbial and nematode communities.     

Carbon source utilization patterns of Bacillus simplex ecotypes do not reflect their adaptation to ecologically divergent slopes in 'Evolution Canyon', Israel

The 'Evolution Canyons' I and II in Israel are model habitats to study adaptation and speciation of bacteria in the environment. These canyons represent similar ecological replicates, separated by 40 km, with a strongly sun-exposed and hot 'African' south-facing slope (SFS) vs. a cooler and mesic-lush 'European' north-facing slope (NFS). Previously, among 131 Bacillus simplex isolates, distinct genetic lineages (ecotypes), each specific for either SFS or NFS, were identified, suggesting a temperature-driven slope-specific adaptation. Here, we asked whether the ecological heterogeneity of SFS vs. NFS also affected carbon utilization abilities, as determined using the Biolog assay. Contrary to expectation, a correlation between substrate utilization patterns and the ecological origin of strains was not found. Rather, the patterns split according to the two major phylogenetic lineages each of which contain SFS and NFS ecotypes. We conclude that traits related to the general energy metabolism, as far as assessed here, are neither shaped by the major abiotic features of 'Evolution Canyon', namely solar radiation, temperature, and drought, nor by the soil characteristics. We further conclude that some traits diverge rather neutrally from each other, whereas other, more environmentally related traits are shaped by natural selection and show evolutionary convergence.     

Variable energetic responses to clutch size manipulations in white-throated dippers Cinclus cinclus

We used the doubly-labelled water technique to measure daily energy expenditure (DEE) of a free-living uniparental incubator, the white-throated dipper Cinclus cinclus , in Scotland. DEE was 205±8 (s.e.m.) kJ d⁻¹ for 17 females incubating their natural clutch sizes, equivalent to 3.2±0.1×basal metabolic rate (BMR). To investigate the influence of clutch size on the energy budget, we measured the DEE of 14 females with clutches increased or reduced by a single egg. Birds with reduced clutch sizes had an energy expenditure with a mean and variance that did not differ from those of birds with unmanipulated clutches. Enlarging the clutch led to an increase in energy expenditure to over 4×BMR for some individuals but not for others, resulting in greater variance in energy expenditure for birds with enlarged clutches. Individual variation in energy expenditure could not be fully explained by environmental conditions, by patterns of behaviour or clutch size. Incubating females received a maximum of only 4 kJ d⁻¹ (2% of DEE) from provisioning by the male, and mobilised up to 6 kJ d⁻¹ (3% of DEE) from reserves. Females spent 2.9±0.2 h (n=20) away from the nest each day, so a foraging rate of 95 kJ h⁻¹ was required during incubation recesses to balance DEE. This 'required foraging rate' is double previous estimates of the maximum rates of energy acquisition for birds of this size. We suggest that the greater likelihood of a raised energy expenditure associated with larger clutches, combined with the difficulties in maintaining energy supplies, may constitute a constraint on avian clutch size.     

Energetic costs of activity by lizards in the field

1. The available data related to the activity energetics of lizards in the field were collated with respect to three indices of activity energetics: the ecological cost of transport (ECT) expressed as a percentage of the total energy, the proportion of total energy used in all forms of activity (%AR), and the sustained metabolic scope (SusMS), defined as the ratio of the total energy expenditure to the total resting metabolism.
2. The ECT values of lizards ranged from 3 to 36% with five of 11 species having values >20%. The percentage AR ranged from 23 to 80% for lizards during active seasons, with most species having values > 50%. The SusMS ranged from 1·1 to 5·1.
3. Values of ECT are higher for lizards than for mammals, in part because the costs of maintenance metabolism are higher in mammals.
4. The percentage AR and SusMS values of mammals are higher than those of lizards.
5. It follows from the previous two points that the proportion of the total daily energy that is expended in non-locomotory activities is disproportionately higher in mammals compared with lizards.
6. The energy expended in locomotion is a significant portion of the energy budget of lizards. This is generally true for all seasons in which there is activity.     

Microsatellite analysis of Drosophila melanogaster populations along a microclimatic contrast at lower Nahel Oren canyon, Mount Carmel, Israel

Drosophila melanogaster populations collected at the south-facing slope (SFS) and north-facing slope (NFS) of lower Nahel Oren canyon, Mount Carmel, Israel display significant differences in survival and longevity at temperature, drought, and starvation stresses. Furthermore, significant assortative mating was previously observed between populations of the two slopes. We used a set of 48 microsatellite markers to analyze patterns of genetic differentiation between D. melanogaster populations from both slopes and D. simulans. Consistent with previous reports, we found D. simulans to be well differentiated from D. melanogaster. Genetic differentiation between SFS and NFS D. melanogaster populations was low (F(ST) = 0.0012). Also a tree of individuals based on the proportion of shared alleles and a model-based clustering method provided no evidence for population substructuring.     

The quantitative genetics of sustained energy budget in a wild mouse

We explored how morphological and physiological traits associated with energy expenditure over long periods of cold exposure would be integrated in a potential response to natural selection in a wild mammal, Phyllotis danwini. In particular, we studied sustained energy expenditure (SusMR), the rate of expenditure fueled by concurrent energy intake, basal metabolic rate (BMR), and sustained metabolic scope (SusMS = SusMR/BMR), a measure of the reserve for sustained work. We included the masses of different central processing organs as an underlying factor that could have a mechanistic link with whole animal traits. Only the liver had heritability statistically different from zero (0.73). Physiological and morphological traits had high levels of specific environmental variance (average 70%) and postnatal common environmental variance (average 30%) which could explain the low heritabilities estimates. Our results, (1) are in accordance with previous studies in mammals that report low heritabilities for metabolic traits (SusMR, BMR, SusMS), (2) but not completely with previous ones that report high heritabilities for morphological traits (masses of central organs), and (3) provide important evidence of the relevance of postnatal common environmental variance to sustained energy expenditure.     

Consumption of Atlantic salmon smolts and parr by goosanders: estimates from doubly-labelled water measurements of captive birds released on two Scottish rivers

The field metabolic rates (FMRs) of nine captive goosanders, Mergus merganser , released on two Scottish rivers were estimated using the doubly-labelled water (DLW) technique. Mean (± S.E.) FMR was 2.322±0.239ml CO₂ g⁻¹ h⁻¹ and daily energy expenditure (DEE) 1939 ± 184 kJ per day. This was significantly greater (x 1.5) than previous estimates which assumed DEE to be three times the basal metabolic rate (BMR) based on regression equations predicting BMR from body mass. FMR of captives and dietary data from previous studies were used to estimate daily consumption of salmon, Salmo salar , smolts and parr by natural populations of these ducks on the river North Esk, north-east Scotland. Goosanders are likely to consume 480-522 g fish per day of which two-thirds are juvenile salmon; equivalent to a daily intake of 10-11 smolts and 48-52 parr. Annual predation of smolts by goosanders was estimated to be between 8000 and 15 000 or 3 and 16% of annual production.     

Biodiversity and microclimatic divergence of chrysomelid beetles at 'Evolution Canyon', Lower Nahal Oren, Mt Carmel, Israel

Ninety species of leaf beetles (Chrysomelidae, Coleoptera, Insecta) were recorded from die 'Evolution Canyon' microsite (sample area 7000 m²), Lower Nahal Oren, Mt Carmel, Israel. Species richness was significandy ( P =0.018) higher on the drier, climatically more fluctuating savanna-like, soudi-facing slope (SFS, 60 species, sample area 3000 m²) than on the cooler, wetter and climatically less variable north-facing slope (NFS, 44 species, sample area 3000 m²), as in other terrestrial taxa. More species were found common to the local xeric SFS at die Canyon and the regional southern desert areas (Judean and Negev Deserts), than to the NFS and Galilee Mountains, respectively, suggesting that local patterns mirror regional patterns. Interslope differences in microclimate variability range indicate that this factor might be determinant of interslope species richness differences. Most species were collected in the spring (April; 47%) and least in the autumn (September; 3%) over the collecting period from March 1995 to June 1997.     

Genome Evolution of the Cyanobacterium Nostoc linckia under Sharp Microclimatic Divergence at "Evolution Canyon," Israel

We describe the genomic DNA diversity and divergence of the cyanobacterium Nostoc linckia from "Evolution Canyon," a microsite consisting of ecologically contrasting slopes, south-facing slope (SFS) and north-facing slope (NFS), at lower Nahal Oren, Mt. Carmel, Israel. The opposing slopes share their limestone lithology but vary greatly in their ecology, primarily because of different levels of solar radiation (which is six times higher on the SFS than on the NFS). The warm and xeric SFS displays a tropical African savanna, whereas the cool and mesic NFS displays a temperate South European Mediterranean live-oak maquis shrub forest. The cyanobacterium Nostoc linckia tested here is a sessile microorganism, growing as a carpet on rock surfaces and constantly exposed to environmental fluctuations of solar radiation, temperature, and desiccation. We demonstrate remarkable interslope and intraslope genetic divergence of the genome (including both coding and noncoding regions) of Nostoc linckia, by using 211 AFLP (amplified fragment length polymorphism) DNA molecular marker loci. Genetic polymorphism of N. linckia subpopulations on the ecologically harsher SFS was significantly (p <0.05) higher (p = 99.53%) than was that of the subpopulations on the climatically milder nfs (p = 85.78%). genetic polymorphism (p) and gene diversity (he) were significantly correlated with variables influencing aridity stress: solar radiation (sr) (rp = 0.956; p = 0.046), temperature (tm) (rp = 0.993; p = 0.0068), and day-night temperature difference (tdd) (rp = 0.975; p = 0.025). as in other tested organisms from "evolution canyon", but even more exceptionally because of its completely sedentary nature, we suggest that the climatically stressed sfs environment is responsible for this marked increase of genetic polymorphism, which is maintained by the combined evolutionary forces of diversifying and balancing selection. This could highlight the importance of ecological stress and selection in evolution and its remarkable effect on the genetic system across the prokaryotic genome.     

Daily energy expenditure in active and inactive persons with spinal cord injury.

Daily energy expenditure (DEE) was evaluated seventeen male subjects with spinal cord injury (SSCI), who had active (N = 9) and inactive (N = 8) lifestyles, and in seven normal males. DEE was estimated from the mean 24-hr heart rate and heart rate-energy expenditure relationship determined in an arm cranking exercise. The DEE of SSCI on active days did not differ from those of normal subjects. On inactive days, SSCI had significantly lower DEE than on active days and than normal subjects. In contrast, the mean 24-hr heart rates of SSCI on active days and inactive SSCI were significantly greater than those of normal subjects, suggesting that SSCI, particularly inactive SSCI, exhibited a slight degree of tachycardia when compared to normal subjects. On inactive days, the DEE was fairly independent of the level of spinal cord injury. However, on active days, there was a clear tendency for SSCI with a low lesion level to have a higher DEE. Even if a SSCI with a high lesion level engaged in active sports, his DEE was relatively small. This lower DEE was largely attributable to the smaller functional muscle mass due to the limitation of physical activity.     

Energy expenditure during egg laying is equal for early and late breeding free-living female great tits

In many bird populations, variation in the timing of reproduction exists but it is not obvious how this variation is maintained as timing has substantial fitness consequences. Daily energy expenditure (DEE) during the egg laying period increases with decreasing temperatures and thus perhaps only females that can produce eggs at low energetic cost will lay early in the season, at low temperatures. We tested whether late laying females have a higher daily energy expenditure during egg laying than early laying females in 43 great tits (Parus major), by comparing on the same day the DEE of early females late in their laying sequence with DEE of late females early in their egg laying sequence. We also validated the assumption that there are no within female differences in DEE within the egg laying sequence. We found a negative effect of temperature and a positive effect of female body mass on DEE but no evidence for differences in DEE between early and late laying females. However, costs incurred during egg laying may have carry-over effects later in the breeding cycle and if such carry-over effects differ for early and late laying females this could contribute to the maintenance of phenotypic variation in laying dates.     

The adaptive significance of insect gall distribution: survivorship of species in xeric and mesic habitats

Summary We studied the relationship between habitat moisture and gall-forming insect populations. Population sizes for most galling taxa were significantly larger in xeric habitats compared with mesic habitats. Our results indicate that the differential abundance of galling insects in these habitats is due primarily to differential mortality and survivorship. Mortality factors acting upon eight insect galling species (belonging to eight genera and four families) were measured on six species (five genera and five families) of host plants. Survival was significantly higher for galling populations inhabiting xeric habitats compared with mesic habitats. Parasitism was higher in mesic habitats in seven of eight habitats and fungus-induced diseases were higher in five of seven habitats. Mortality due to predation and other (unknown) factors showed no clear trends. Overall, there was a tendency towards lower mortality and consequently higher survival for populations inhabiting xeric habitats. We hypothesize that reduced mortality caused by natural enemies and endophytic fungi has contributed to the speciation and radiation of galling insects in apparently harsh environments.     

Competition between Eurasian red and introduced Eastern grey squirrels: the energetic significance of body-mass differences

Daily energy expenditure (DEE) was measured in sympatric populations of red and grey squirrels using the doubly labelled water technique. Grey squirrels had significantly higher DEEs than red squirrels. However, the difference between the species was not separable from the effects of body mass on DEE. The DEEs of both species were in accordance with published allometric predictions incorporating body mass and ambient temperature. The differences in energetic requirements and social dominance, both consequences of body size, may represent means by which grey squirrels exert more interspecific competition on red squirrels than do conspecifics, potentially driving populations below viable levels in some sites.